#box{
    width: 500px;
    height: 500px;
    border: 1px red solid;
    position: relative;
    text-align: center;
}
.minbox{
    width: 100px;
    height: 100px;
    position: absolute;
    left: 50%;
    top: 50%;
    margin-left: -50px;
    margin-top: -50px;
    border: 1px black solid;
}